The present invention relates to a shaft assembly for use with mechanisms or systems for applying mechanical load along the length of a thermal print head of a thermal printing system.
Within the broader category of electronic printer products, several different marking technologies have been employed to create and fix images to flexible print media such as paper, film and the like. Three contemporary technologies that are frequently used include xerographic, ink-jet and thermal printing. As is true for any of these technologies, thermal printing requires the implementation of hardware specific to its technology in order to accomplish the task of the thermal printing process.
A fundamental piece of hardware specific to thermal printing technology is the thermal print head. A thermal print head typically is in the form of a printed circuit board, incorporating several very small resistive heating elements positioned in a uniformly close spaced linear array, altogether comprising a print line which resides along or near one edge. The circuit board may be bonded to an aluminum support block to increase its structure rigidity.
During normal printer operation, the print head ordinarily is positioned so that the print line tends toward tangential contact with the outer cylindrical surface of an opposing platen roll and may be pivoted away from the platen roll for printer set-up or servicing. A spring load is typically applied to the print head to compliantly bias the print line in a direction normal to the platen roll surface. Media typically supplied by a spool is held in pressure contact, sandwiched between the print line and platen roll, while the platen roll is rotationally driven and print line resistive heating elements are selectively activated, in order to produce the desired two dimensional image. The area of pressure contact developed by the print line forcibly acting through the media and on to the platen roll is often referred to as the print nip.
Compressive stress within the print nip must be adequate to produce intimate contact between the print line resistive heating elements and media and thus guarantee thermal energy transfer for proper image formation, otherwise areas void of image may result. Conversely, excessive compressive stress within the print nip can cause increased abrasive wear on the thermal print head, resulting in premature print head degradation and diminished service life.
In certain types of thermal printers, it is desirable to have the capability to process a variety of different media sizes and therefore vary widths of media, all positionally registered from one end of the print head. As the width of the media being printed varies so does the effective area of the print nip. To maintain an optimum level of compressive stress, the print head load should generally vary with the area over which the load is distributed. Also, the load is distributed over the region of the print head under which the media resides, to prevent uneven contact stress from one end of the print nip to the other.
One prior art mechanism for applying a spring load to a thermal print head uses a linear plunger device incorporating a compression spring and adjustment nut. The adjustment nut can be rotated to vary the deflection of the compression spring and thereby change the plunger load against the print head. One or more of these linear plunger devices may be positioned to selectively engage a print head at a range of positions along its length. One drawback to this prior art mechanism is that for any particular width media and subsequent print nip area, there is no established print head load or position of load predetermined by engineering design. An individual setting up the machine typically must guess at load settings and position of the linear plunger devices, then run the printer to determine if print quality is acceptable. This process is iterative by nature, can take considerable time to achieve acceptable print quality, and can result in the waste of a substantial quantity of print media. Also, the linear plunger devices occupy a rather large space and are cumbersome to move in order to raise the print head for printer set-up or servicing.
Accordingly, it is an object of the present invention to provide a shaft assembly for a thermal printer system that enables the thermal print head to readily accommodate media of different widths.
It is a further object of the present invention to provide such a shaft assembly that is adapted to adjustably apply loads along the length of the thermal print head of the thermal printer system to accommodate the desired width of media.
It is a further object of the present invention to provide such a shaft assembly that includes a shaft and a plurality of cam mechanisms mounted thereto, each cam mechanism adapted to be rotated to apply a respective load to structure associated with the thermal print head to increase the load applied to the thermal print head along the length of the thermal print head to facilitate accommodation of wider media.
It is a still further object of the present invention to provide such a shaft assembly wherein the cam mechanisms are rotatably coupled to each other such that rotation of the shaft a predetermined amount causes one cam mechanism to rotate to a load-applying position, and further incremental rotations of the shaft causes additional cam mechanisms to rotate to their respective load-applying positions to thereby increase the load applied to the structure associated with the thermal print head and thereby facilitate accommodation of wider media.
In accordance with these and other objects, the present invention is directed to a shaft assembly for a thermal printer system for adjustably applying loads along the length of a thermal print head of the thermal printer system. The shaft assembly includes a shaft, and a plurality of cam mechanisms rotatably mounted on the shaft and adapted to apply respective loads to the thermal print head at respective locations along the length of the thermal print head. Each cam mechanism is rotationally coupled with an adjacent cam mechanism, and is rotatable from a non-load-applying position to a load-applying position and from the load-applying position to subsequent load-applying positions. Each cam mechanism is adapted to apply a respective load to structure associated with the thermal print head at a respective location of the structure when the cam mechanism is in its load-applying position and when the cam mechanism is in any of its subsequent load-applying positions. Each cam mechanism engages the adjacent cam mechanism as the cam mechanism rotates from its load-applying position to a first of its subsequent load-applying positions to cause the adjacent cam mechanism to rotate from the non-load applying position of the adjacent cam mechanism to the load-applying position of the adjacent cam mechanism. The cam mechanisms desirably are rotationally coupled, such that each incremental rotation of the shaft in a first or loading direction causes an additional cam mechanism to rotate from its non-load applying position to its load-applying position. The plurality of cam mechanisms may include any suitable number of cam mechanisms.
The cam mechanisms desirably all have the same construction, including any suitable cam profile. In a preferred embodiment, for example, each cam mechanism comprises a single lobe profile; and the degree of rotation of each cam mechanism from its non-load-applying position to its load-apply position is 180xc2x0 and the degree of rotation from its load-applying position to subsequent load-applying positions is a multiple 360xc2x0. In accordance with an alternative embodiment, each cam mechanism may instead comprise a double lobe profile; and the degree of rotation of each cam mechanism from its non-load-applying position to its load-applying position is 90xc2x0 and the degree of rotation from its load-apply position to subsequent load-apply positions is a multiple of 180xc2x0. In accordance with further alternative embodiments, the cam mechanisms may have other suitable profiles.
The cam mechanisms may be rotationally coupled together in a generally side-by-side manner in any suitable manner. For example, in accordance with a preferred embodiment, the shaft assembly may include a plurality of coupling members for coupling together the cam mechanisms, preferably with each coupling member coupling together a respective pair of cam mechanisms. Desirably, each cam mechanism includes a pair of end faces, and a nub on each end face, and each coupling member defines a pair of slots, each slot receiving one of the nubs of a respective cam mechanism. Desirably, the nubs and slots are arcuate, and the arc lengths of the nub and slots depend on the profile configuration of the cam mechanisms. If desired, the cam mechanisms may instead be rotationally coupled directly to each other in accordance with alternative embodiments of the invention.
The end cam mechanism may be rotated manually or in any suitable mechanical or electronic manner to cause rotation of the cam mechanisms. In a preferred embodiment, for example, the shaft assembly includes a drive hub rotationally coupled with one of the end cam mechanisms and a one-way roller clutch bearing associated with the drive hub so that rotation of the shaft in the first direction rotates the end cam mechanism to the load-applying position.
In a preferred embodiment, the shaft assembly also includes an unloading drive hub rotationally coupled with the other end cam mechanisms, and an other one-way roller clutch bearing for causing rotation of the cam mechanisms to non-load applying positions, preferably one at a time in response to rotation of the shaft in a second or unloading direction. The shaft assembly preferably includes a drive assembly that includes the unloading drive hub and a gear assembly for rotating the unloading drive hub in the first direction as the shaft rotates in the second direction. The gear assembly may include a drive hub gear associated with the unloading drive hub and a gear clutch disposed about the shaft, and a pair of idler gears which cause the drive hub to rotate in the first direction when the shaft is rotated in the second direction. The gear clutch desirably also includes a one-way roller clutch bearing oriented in a direction opposition the orientation of the roller clutch bearing associated with the drive hub.
Accordingly, in a preferred embodiment, the rotation of the shaft in the first direction causes driving load engagement of the one-way roller clutch bearing associated with the drive hub and therefore rotates the drive hub in the first or loading direction while the second one-way roller clutch bearing and its associated gear clutch are free from driving load engagement. The drive hub being rotationally coupled to the end cam mechanism in turn causes rotation of that adjacent cam mechanism in the first direction. Rotation of the shaft in second or unloading direction causes driving load engagement of the other one-way roller clutch bearing associated with the gear clutch and therefore rotates the gear clutch in the first direction while the drive hub and its associated one way roller clutch bearing are free from driving load engagement. The unloading drive hub is rotationally coupled to the other end cam mechanism and, because of the gear assembly, causes rotation of that other end cam mechanism in the first direction when the shaft is rotated in the second direction.
Accordingly, the present invention in accordance with a preferred embodiment provides a shaft assembly for a thermal printer system readily adapted to produce an appropriate predetermined force, properly distributed along the length of the print head, for the desired width media. A first turn or rotation of the shaft produces a predetermined load, appropriate for narrow width media, positioned adjacent the first end of the print head. Subsequent rotations of the knob in the same direction produce additional loading of the print head progressing from the first and toward the second end by rotating and engaging cams one by one, to accommodate increasingly wider media. Additionally, desirably, rotation of the shaft in the opposite direction disengages the loading of the print head toward the first end by rotating and engaging the cams one by one in a reverse direction. Thus, in accordance with a preferred embodiment, the printer can be set-up to process different width media very quickly and without waste of media.